How is the total number of electrons represented in a Lewis structure determined?
How else but by counting up (i) the number of valence electrons on the neutral atoms, and (ii) the charge on the atom or ion............?
Now we build Lewis structures by elaborating from neutral atoms, and of course, we have to account for the charge on the atom or radical ion.
Take for example
And thus we get
So all we do is to take the number of valence electrons (which is given by the atom's Group number), and add these numbers together, and add or subtract depending on the negative or positive charge of the species.
Can you do the same for